If ignore_auth_case is True, username conversion to lowercase is required.
I guess, that adding a corresponding hint to the registration page will avoid
confusion about this conversion of all user input for the username field.

The longer I thought about it, the more I've been convinced, that a username
differing only by character case is spelling trouble for any environment.
I remembered having such problems when switching ignore_auth_case to True
for a Trac instance some time ago and couldn't think of a use-case anyway,
so I made case-less checks mandatory.

Even better, pulling the obsoleted new option now is fighting feature bloat:
With 25+ options AccountManager is not exactly easy to configure, so every
dispensed option is a blessing.

Additionally I hope, that a highlighted username improves error messages.

Refactored code to reduce amount of repetitive work in creating and filtering the set of wiki pages. The work of creating the set is now done on initialization, and the IWikiChangeListener methods are solely responsible for updating the set.

There's still no visible functionality apart from an empty admin panel and a
newly added permission. However, now 4 real unit tests replace 2 of the dummy
ones to ease validation of plugin behavior in the future.

Still I'm a bit lazy here, because for 100% code coverage I should probably
have added a functional test for the admin panel as well. Let's postpone this
to a later time, when there's more real content to check for.

Consider unit tests to be essential for QA of non-trivial stuff, so include
it here right from the start. Thanks to Odd Simon for a recent example
implementation in TagsPlugintrunk, that served as coding style guide to me.

Refs #9912: applied proposed patch of Micha Glave, but changed it a bit, because for compat reasons, table column 'summary' is added on upgrade from version 3 to 4. Please, test this commit and report if it still works with MySQL.